Radial and tangential polarizers

ABSTRACT

Radial and tangential polarizers selectively attenuate meridional or skew rays in a focusing waveguide. A radial polarizer is a polarizer in which the direction of polarization at any point in the polarizer is approximately parallel to a radial line from the center of the polarizer. A tangential polarizer is a polarizer in which the direction of polarization at any point in the polarizer is approximately perpendicular to a radial line from the center of the polarizer. A pair of radial polarizers or a pair of tangential polarizers spaced apart at an odd multiple of one-fourth of the period of oscillation of a meridional ray about the center of the optical path attenuate skew rays in favor of meridional rays. A radial polarizer and a tangential polarizer similarly spaced attenuate meridional rays in favor of skew rays.

United States Patent [1 1 [111 3,719,415

Rawson 1 March 6, 1973 [5 RADIAL AND TANGENTIAL Primary Erarniner- DavidSchonberg POLARIZERS Assistant Examiner-Paul R. Miller [75] Inventor:Eric Gordon Rawson, Stirling, NJ. Attorney-R Guemher et [73] Assignee:Bell Telephone Laboratories, lncor- [57] ABSTRACT t M H'll, N.

Po" my 1 J Radial and tangential polarizers selectively attenuate Filed!q 1971 meridional or skew rays in a focusing waveguide. A

Appl. No.: 182,741

radial polarizer is a polarizer in which the direction of polarizationat any point in the polarizer is approximately parallel to a radial linefrom the center of the polarizer. A tangential polarizer is a polarizerin which the direction of polarization at any point in the polarizer isapproximately perpendicular to a radial line from the center of thepolarizer. A pair of radial polarizers or a pair of tangentialpolarizers spaced apart at an odd multiple of one-fourth of the periodof oscillation of a meridional ray about the center of the optical pathattenuate skew rays in favor of meridional rays. A radial polarizer anda tangential polarizer similarly spaced attenuate meridional rays infavor of skew rays.

6 Claims, 4 Drawing Figures RADIAL AND TANGENTIAL POLARIZERS BACKGROUNDOF THE INVENTION This concerns optical apparatus and in particularapparatus for selectively reducing aberrations in focusing elements.

The light propagating in optical systems may be classified generally asmeridional rays and skew rays. Meridional rays propagate through thesystem in a direction that can be resolved into one component that isparallel to the optical axis of the system and a second component thatis perpendicular to it. For these rays there is no third componentperpendicular to the other two. Skew rays however, propagate in adirection that has three mutually perpendicular components. Focusingelements typically exhibit different aberrations for these two types ofrays. Accordingly, it is frequently desirable to find some means forselectively attenuating one or the other type of ray.

This is particularly desirable in optical waveguides used to transmitlight over extremely long distances, as is necessary for high-capacityoptical communication systems. One such waveguide includes a successionof conventional lenses spaced regularly along the waveguide, each ofwhich focuses propagating light. In another type of waveguide,the-conducting path is a continuous optical fiber. The index ofrefraction in this optical fiber decreases with distance from theoptical axis of the fiber with the result that the optical fiber hasproperties similar to those of the waveguide having discreteconventional lenses. Such fibers are referred to as graded index (GRIN)or SELFOC fibers, the latter name being a registered trade name of theNippon Electric Company and the Nippon Sheet Glass Company. Furtherdetails may be found in the paper by E. G. Rawson, D. R. I'Ierriott andJ. McKenna on Analysis of Refractive Index Distributions in Cylindrical,

Graded-Index Glass Rods (GRIN Rods) Used as Image Relays Applied Optics,Vol. 9, No. 3, (March I970) and in the British patent of T. Uchida andM. Sakaguchi N0. l,233,653.

In optical waveguides such as these, any propagating light ray will beobserved to oscillate spatially about the optical axis. Becausemeridional rays have only one component perpendicular to the opticalaxis, the oscillation of a meridional ray is an oscillation that carriesit through the optical-axis of the waveguide. On the other hand. skewrays travel in more complex paths and do not pass through the opticalaxis. In general, the path of such rays is an elliptical helix with onesub-class of SUMMARY or rua INVENTION Accordingly, it is an object ofthis invention to selectiveiy attenuate either meridional rays or skewrays that Propagate through optical waveguides.

It is a further object of this invention to reduce the aberrationsinherent in light propagation through graded index glass rods (GRINrods).

These and other objects of my invention are achieved by the use ofpolarizers that may be referred to as radial and tangential polarizers.A radial polarizer is a polarizer in which the direction of polarizationat any point in the polarizer is approximately parallel to a radial linefrom the center of the polarizer. A tangential polarizer is one in whichthe direction of polarization at any point in the polarizer isapproximately perpendicular to a radial line from the center of thepolarizer.

In accordance with the invention, either meridional rays or skew raysare attenuated by placing a succession of polarizers in the opticalwaveguide, each separated by an odd multiple of one-fourth the period ofoscillation of a meridional ray about the optical axis of the waveguide.The polarizers are centered on the waveguide so that their centerscoincide with the optical axis of the waveguide. To attenuate skew rayswhile passing meridional rays, I place one or more pairs of radialpolarizers in the optical waveguide, each polarizer separated by aspacing that is an odd multiple of a quarter of the period ofoscillation of a meridional ray about the center of the optical path.Alternatively, skew rays may be attenuated by placing one or more pairsof tangential polarizers in the waveguide at the same spacing. Toattenuate meridional rays in favor of skew rays,.l place one or morepairs of polarizers in the waveguide, each pair consisting of atangential and a radial polarizer spaced apart by an odd multiple of aquarter of the period of oscillation of a meridional ray about thecenter of the optical path. As will be explained later, the extent ordegreeof attenuation can be controlled.

Because either the skew rays or the meridional rays may be selectivelyand controllably attenuated, it is now possible to reduce, by methodswell understood in the art, aberrations in the waveguide. Consequently,a substantially aberration-free output can be obtained from the opticalwaveguide.

BRIEF DESCRIPTION OF THE DRAWING These and other elements, features andobjects of my invention will be more readily understood in the followingdetailed description of my invention taken in conjunction with thefollowing drawing in which:

FIG. 1 is a schematic illustration of the propagation of a light raythrough a conventional waveguide;

FIG. 2 is a schematic illustration of a radial polarizer according to myinvention;

FIG. 3 is a schematic illustration of a tangential polarizer accordingto my invention; and

FIG. 4 is a schematic illustration of the use of the polarizers of FIGS.2 and 3 in a waveguide to attenuate selectively the propagation ofeither meridional rays or skew rays.

DETAILED DESCRIPTION OF THE DRAWING In FIG. 1 there is shown a typicalwaveguide 11 that uses a series of conventional lenses 13 to guide thepropagation of light along an optical axis 0A. In this figure the pathtraced by a light ray 15 is also shown. Note that ray l5 oscillatesabout axis 0A.

As indicated above, the rays that propagate through a waveguide can beclassified as meridional and skew rays. Because the aberrations of awaveguide differ for these two types of rays, I have found it desirableto selectively attenuate one type of ray in favor of the other. Thisattenuation is accomplished by using radial and/or tangentialpolarizers.

A schematic illustration of an illustrative radial polarizer 21 of myinvention is shown in FIG. 2. The center of polarizer 21 is indicated at22. Throughout the polarizer the direction of polarization as indicatedby arrows 24 is everywhere approximately parallel to a radial line fromthe center 22 of the polarizer.

A schematic illustration of an illustrative embodiment of a tangentialpolarizer 31 having a center 32 is shown in FIG. 3. As indicated byarrows 34, the direction of polarization at each point 33 is everywhereapproximately perpendicular to a radial line from the center 32 oftangential polarizer 31.

Polarizers 21 and 31 are used to selectively attenuate skew rays andmeridional rays in illustrative a'pparatus depicted in FIG. 4. Thisapparatus comprises an optical waveguide 41 in which light is propagatedalong optical path A. Preferably this optical waveguide is a gradedindex glass rod in which individual light rays propagate along paths ina fashion described in the aforementioned Applied Optics paper. Inparticular, meridional and skew rays will both be observed to oscillateabout the optical axis, with meridional rays following paths that passdirectly through the optical axis, and skew rays following ellipticallyhelical paths at either fixed or variable distances from the opticalaxis.

To attenuate skew rays, I place two or more polarizers at spacings thatare an odd multiple of onequarter of the period of oscillation of ameridional ray about optical axis 0A, the polarizers all being eitherradial or all tangential polarizers. Thus, in FIG. 4 there are shownfour radial polarizers 21 located at spacings that are a quarter theperiod of oscillation of a meridional ray 45. Attenuation of the skewrays could be achieved using only a pair of polarizers 21; but with alarger number of polarizers, it is possible to achieve even greaterattenuation. Alternatively, the degree of attenuation can be controlledby controlling the density of dichroic crystals within each polarizer.The same attenuation of a skew ray could also be achieved bysubstituting tangential polarizers for the radial polarizers shown inFIG. 4

to attenuate meridional rays in favor of skew rays, I use one or morepairs of polarizers, each pair consisting of a radial and a tangentialpolarizer spaced apart a distance equal to an odd multiple of a quarterof the period of oscillation of the meridional ray. Thus, with referenceto FIG. 4, I attenuate meridional rays by substituting a tangentialpolarizer 31 for every other radial polarizer there shown. Since radialand tangential polarizers both give linear polarization in which thefield vectors are circularly symmetric about a point, they may be knownas circular symmetric linear polarizers.

Radial and tangential polarizers 21 and 31 may be made in several ways.They can, for example, be manufactured by cutting conventional linearpolarizers such as Polaroid polarizers and fitting the pieces togetherso that the direction of polarization conforms to the directionsdescribed above.

They may also be formed by various drawing operations similar to thoseused in manufacturing polarizers such as Polaroid polarizers. In thisprocedure, a suspension of dichroic crystals such as herapathitecrystals is formed in a transparent matrix. A radial polarizer is thenmade by drawing this suspension from the center; this aligns thecrystals such that they pass light polarized in a directionapproximately parallel to a radial line from the location at which thedrawing is made.

A tangential polarizer may be formed by rotating the suspension ofherapathite crystals so that the crystals become aligned in a directionsuch as to pass light having a direction of polarization approximatelyperpendicular to a radial line from the center of rotation.

As will be obvious to those skilled in the art, my invention may be usedin any situation where it is desired to attenuate meridional or skewrays. Numerous techniques will also be evident for forming thepolarizers I have described. In particular, it would be evident to varythe density of dichroic crystals in the radial direction to controllablyattenuate skew rays as a function of degree of skew. Still otherarrangements that fall within the skill and scope of this invention willbe apparent to those skilled in the art.

What is claimed is:

1. Optical transmission apparatus comprising:

means for guiding light along a path;

said guiding means comprising means including focusing means for causinglight rays to oscillate with distance about the center of the path;

means for preventing light from propagating along the guiding means bothas meridional rays and as skew rays comprising a plurality of polarizingdevices centered on the path;

said polarizing devices being spaced apart by a distance equal toapproximately an integral odd multiple of one-fourth of the period ofoscillation of the meridional ray about the center of the path.

2. The optical transmission apparatus of claim 1 wherein:

the polarizing devices are circularly symmetric linear polarizers.

3. The optical transmission apparatus of claim 1 wherein at least twopolarizing devices are radial polarizers, whereby skew rays areattenuated.

4. The optical transmission apparatusof claim 1 wherein at least twopolarizing devices are tangential polarizers, whereby skew rays areattenuated.

5. The optical transmission apparatus of claim 1 wherein one polarizingdevice is a radial polarizer and another polarizing device is atangential polarizer, whereby meridional rays are attenuated.

6. The optical transmission apparatus of claim 1 wherein thedistribution of skew rays and meridional rays is optimized by selectivecontrolled attenuation, to permit the focusing means to be corrected soas to minimize aberrations.

1. Optical transmission apparatus comprising: means for guiding lightalong a path; said guiding means comprising means including focusingmeans for causing light rays to oscillate with distance about the centerof the path; means for preventing light from propagating along theguiding means both as meridional rays and as skew rays comprising aplurality of polarizing devices centered on the path; said polarizingdevices being spaced apart by a distance equal to approximately anintegral odd multiple of one-fourth of the period of oscillation of themeridional ray about the center of the path.
 1. Optical transmissionapparatus comprising: means for guiding light along a path; said guidingmeans comprising means including focusing means for causing light raysto oscillate with distance about the center of the path; means forpreventing light from propagating along the guiding means both asmeridional rays and as skew rays comprising a plurality of polarizingdevices centered on the path; said polarizing devices being spaced apartby a distance equal to approximately an integral odd multiple ofone-fourth of the period of oscillation of the meridional ray about thecenter of the path.
 2. The optical transmission apparatus of claim 1wherein: the polarizing devices are circularly symmetric linearpolarizers.
 3. The optical transmission apparatus of claim 1 wherein atleast two polarizing devices are radial polarizers, whereby skew raysare attenuated.
 4. The optical transmission apparatus of claim 1 whereinat least two polarizing devices are tangential polarizers, whereby skewrays are attenuated.
 5. The optical transmission apparatus of claim 1wherein one polarizing device is a radial polarizer and anotherpolarizing device is a tangential polarizer, whereby meridional rays areattenuated.